Among social insects, ants constitute one of the main pests that adversely affect human activities. For instance, leaf-cutting ants of the genera Atta and Acromyrmex (Order Hymenoptera, Family Formicidae, Subfamily Myrmecinae, Tribe upper Attini) are endemic of the Neotropics. These ants cut pieces of fresh plant matter (leaves, flowers, fruits), which are then transported to the ant nests and used as substrate for growing of the symbiotic Leucoagaricus fungus that these ants use as their main source of food. Leaf-cutting ants are the main herbivores of the Neotropics, and their impact in agriculture has been estimated in billions of US dollars every year, both by directly attacking crops and plantations and by competing for pastures with livestock (Robinson S. W. and H. G. Fowler (1982). Foraging and pest potencial of paraguayan grass-cutting ants (Atta and Acromyrmex) to the cattle industry. Z. Angew. Ent 93: 42-54; Hölldobler, B. and Wilson, E. O. (1990) The ants, Harvard University Press. Cambridge, Mass., USA; Della Lucia T. M. C. (ed.) (1993). As formigas cortadeiras. Bib. Cent. Univ. Fed. Viçosa Brasil.). Beyond the economic loss infringed every year for its damage to plantations of native and exotic species, to crops of citrus, yerba mate, and vegetables, people perceives them negatively because they destroy ornamental plants in the gardens.
Combat of social insects, as ants, presents challenges that are different from controlling solitary organisms. For instance, there is a difference in the concentration of individuals of the pest species (a single organism versus thousands to millions of individuals in a colony). Furthermore, in social insects the queen is the only colony member that reproduces, and is very well protected. As an example, in leaf-cutting ants the queen is the first one to escape in case of danger, carrying with her a piece of the Leucoagaricus fungus that these ants use as source of food and founding a new colony at a different site.
The fact that leaf-cutting ant colonies build their nest well protected underground makes it difficult under usual conditions to directly apply poisons or pesticides which could result in an effective control of the whole colony. With this in mind, the more efficient control is achieved by insufflating a controlling agent through every entrance/exit of the nest. However, this procedure is labor-consuming and requires the use of special equipment, which raises costs, and even then it can be ineffective as the controlling agent might not reach the queen due to the large underground dimensions and the numerous chambers that a giant nest may have. For countering this problem, advantage has been taken of the characteristic behavior of the leaf-cutting ants of carrying pieces of leaves to the nest interior by using baits impregnated with pesticides of long-lasting residual effect, which are carried by the ants to places otherwise inaccessible within the nest, thus affecting the health of the colony in a more efficient way. Nevertheless, social insects, and especially leaf-cutting ants, also present complex cleaning behavior. Leaf-cutting ants can learn to recognize a particular food item, and as a result come back for more or reject it (Knapp J, Howse P. E y A. Kemarrec. 1990. Factors controlling foraging patterns in the leaf-cutting ant Acromyrmex octospinosus. In Applied Myrmecology: a world perspective, Eds. R. K. Vander Meer and K. Jaffé, pp 382-409, Boulder, Colo.: Westview Press; Herz H, Höldobler B and F. Roces. 2008. Delayed rejection in a leaf-cutting ant after foraging on plants unsuitable for the symbiotic fungus. Behavioral Ecology 19: 575-582; Saverscheck N, Herz H, Wagner N y F. Roces. 2010. Avoiding plants unsuitable for the symbiotic fungus: learning and long-term memory in leaf-cutting ants. Animal Behaviour 79: 689-698). Regarding rejection, workers can inspect a food source and leave it without carrying it, they can start to eat or carry it just for leaving abandoned afterwards, they can carry the resource to the nest but reject it at subsequent times, or they can carry a same resource to the nest several times but at a decreasing rate in time. It has been also demonstrated that these ants can associate. Thus, leaves usually attractive for the ants, impregnated in a fungicide which affects the Leucoagaricus gardens may be carried to the nest at first, but then will appear in the colony's refuse dump, as these leaves are removed from the fungus growing chamber. Moreover, when baits infected with a mycopathogenic fungus are accepted, the ants later reject the same bait, even if it is not infected with the pathogen. This memory-associated rejection can start 24-48 hours after the damaging resource is offered and last from 18-30 weeks. Because the effective control of a pest often requires repeated exposure of the target organism to the controlling agent, the delayed rejection mechanism displayed by leaf-cutting ants reduces the efficiency of using baits for fighting them (Ridley P, Howse P. E. and C. W. Jackson. 1996. Control of the behavior of leaf-cutting ants by their symbiotic fungus. Experientia 52: 631-635).
Traditionally, social insect's pest control has relied upon the use of chemical pesticides. Several well-known problems are associated to the use of pesticides, such as negative impact on human health, persistence in the ecosystem, and lack of specificity, incidentally adversely affecting many non-damaging organisms, including those which could be beneficial for combating the very same pest whose control is desired. The use of many pesticides of common use in the past has been banned by local authorities around the world for the risk that they pose to the people and the environment, and many other became ineffective as pest populations developed resistance and became immune to them. Moreover, chemical control of ants is highly inefficient since the queen is seldom adversely affected and then the colony being fought can relocate to a nearby location.
An alternative to the use of pesticides is biological control of pests. Biological control, or biocotrol, utilizes the natural enemies of the pest for controlling its populations. For instance, fungi which are insect pathogens have been used for controlling unwanted insects, as disclosed in Patent Applications WO9102051, WO9424871 WO9525430, WO0228189, WO2006121350 y WO2004052103, and in U.S. Pat. No. 7,951,389 y U.S. Pat. No. 6,403,085. Use of fungi which are pathogens of other fungi is discussed in WO016974 y WO9618722A2, which use strains of Trichoderma for combating phytopathogenic fungi, and there is even a commercial product (Attacebo) which uses Trichoderma for destroying the Leucoagaricus gardens grown by leaf-cutting ants. However, biological control usually requires repeated exposure of the target organism to its pathogen, and the delayed rejection mechanism makes the use of baits as carriers in methods of biological control of leaf-cutting ants particularly inadequate because their memory and associative ability.
From the foregoing it is clear that new methods for fighting leaf-cutting ants are needed which eliminate them in spite of the protection provided by ant nests. In particular, methods for fighting leaf-cutting ants by using baits are needed whose efficiency is not hindered by the delayed rejection mechanism.